Make terrain brushes not overwrite already placed terrain unless using super-force-build

[Orum]

It is extremely annoying that using one terrain brush on terrain that has already been placed overwrites the older terrain, even when not using either of the force-build placement modes. It's primarily a problem on Gleba. Instead, it should only overwrite already placed terrain when using super-force-build.

As an added bonus, it would greatly reduce if not eliminate the frequent times I encounter bug 122905.